It is known for a nozzle in hot runner injection molding apparatuses to include a thermally conductive body portion, a thermally conductive head portion and a heater. Typically, the head portion has a first diameter and the body portion has a second, smaller diameter. A shoulder is the transition region between the head and body portions. The head portion of the nozzle typically seats against a manifold block so that the nozzle can receive melt from runners in the manifold block. Typically, the shoulder abuts a spacer, which in turn, abuts a mold block that contains a plurality of mold cavities. The mold block is typically maintained at a temperature that is significantly below the temperature of the nozzle, in order to cool and solidify melt in the mold cavities.
The heater for the nozzle may be in the form of a wire that is wrapped around the body portion. The heater is typically not wrapped around the head portion. The nozzle however, can lose heat to the mold block through the contact between the head portion and the mold block by means of the spacer. As a result, the temperature of the melt in the head portion is lower than the temperature of the melt at other points along the length of the nozzle. It is generally preferred to have as uniform a temperature as possible for melt along the length of the nozzle, because it reduces the risk of inadvertently burning or otherwise degrading melt in a hotter portion of the nozzle, or of overcooling the melt, which can cause melt flow difficulties, in a cooler portion of the nozzle.
Some nozzles have been proposed to raise the temperature of the melt in the head portion. For example, U.S. Pat. No. 4,771,164 discloses a nozzle having a body portion and a separate head that can be attached to the body portion. A heater wire is wrapped along the length of the body portion including the portion proximate the top end thereof. The head is then attached to the body portion, covering some of the wrapped wire heater. In this way the heater is able to heat the portion of the nozzle melt channel in the top end of the body portion.
There is a continuing need, however, for nozzles having improved heating systems.